feat(uat-batch-19): a11y th scopes + legend styling + i18n locale fixes

- Raw `<th>` cells gain `scope="col"` so SR users get proper column
  association: berth-interests-tab, bulk-add-berths-wizard,
  clients/bulk-hard-delete-dialog. shadcn `<TableHead>` migration
  would be cleaner but the scope attribute is the minimum-effort fix
  the queue's a11y entry asks for.
- supplemental-info form `<legend>` elements styled with
  `mb-2 px-1 font-semibold` so they read as section headings rather
  than blending into the surrounding fieldset border (default browser
  legend rendering is barely visible).
- payments-section: invalid `'en-EU'` BCP-47 locale → `undefined` to
  honour browser locale.
- ui/calendar: literal `'default'` → `undefined` on the month
  dropdown formatter, same reason.

tsc clean. 1419/1419 vitest pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/berths/berth-interests-tab.tsx

@@ -146,12 +146,22 @@ export function BerthInterestsTab({ berthId }: BerthInterestsTabProps) {
         <table className="w-full text-sm" data-testid="berth-interests-table">
           <thead className="bg-muted/40 text-left text-xs font-medium text-muted-foreground">
             <tr>
-              <th className="px-3 py-2">Client</th>
-              <th className="px-3 py-2">Stage</th>
-              <th className="px-3 py-2">Category</th>
-              <th className="px-3 py-2">Source</th>
-              <th className="px-3 py-2">Last activity</th>
-              <th className="px-3 py-2 text-right" />
+              <th scope="col" className="px-3 py-2">
+                Client
+              </th>
+              <th scope="col" className="px-3 py-2">
+                Stage
+              </th>
+              <th scope="col" className="px-3 py-2">
+                Category
+              </th>
+              <th scope="col" className="px-3 py-2">
+                Source
+              </th>
+              <th scope="col" className="px-3 py-2">
+                Last activity
+              </th>
+              <th scope="col" className="px-3 py-2 text-right" />
             </tr>
           </thead>
           <tbody>